1. Field of the Invention
This invention relates to digital imaging devices and methods, and more particularly to multimode scanning of an image sensor.
2. Description of the Related Art
Imaging sensors are used in digital cameras and camcorders, high definition televisions (HDTV), telescopes and other equipment. Two types of image sensors commonly used for these applications are charge coupled device (CCD) and complementary metal oxide semiconductor (CMOS). Each includes a detector portion, typically a two-dimensional array of pixel circuits. Each pixel circuit contains both a detector that converts photons (electromagnetic radiation) into a charge (electron-hole pairs), that accumulates at the detector, and an output circuit. Each detector has a maximum charge that it is capable of holding. When this maximum charge has accumulated, the detector saturates and cannot hold any more.
Each pixel senses one small area within the larger image, with its circuit outputting a signal representing that portion of the image. The pixel circuits may require resetting to obtain a new image or to accommodate a bright star that has saturated the circuit.
Image sensing for astronomy applications currently requires two image sensors to record an image of the sky. One sensor is used to fix the telescope orientation with respect to a “guide star” as the earth rotates, and another to sense the image. The guide star is typically a bright star that can be easily tracked. Because a bright star is used, the guide star sensor quickly saturates and must be reset more frequently than the image sensor. Also, for accurate tracking, a high frame rate is required for the guide star sensor.
Most imaging sensors read out and reset rows or columns of pixels at a time. This makes it difficult to concentrate on only one portion of the overall image.